The present invention concerns a low capacity and high power density battery apt to supply high power output for short periods of time and therefore particularly suitable as a back-up to a steady-output generator, for electric vehicles in the urban traffic.
Electric traction is considered a possible solution to the problems of air and noise pollution. Two lines of development have been pursued so far, the first one resulting in some practical applications in the field of public vehicles (mail and milk distribution, warehouse fork-lifts etc.), provided with an electric battery which is recharged at the end of the day. Presently, lead batteries with a specific energy density of 20-30 Wh/kg ensure a range of operation (an autonomy distance span) of 100/200 km, but weight, even for the so-called "Iron-clad" types, is a serious drawback especially in town traffic where the acceleration necessary to cope with the dynamics of the traffic involves significant power outputs. Many solutions have been tested in an effort to optimize the autonomy-weight-power relationship but without effective results.
Attempts have been made also in the field of light batteries, zinc-air for instance, which even when reaching high energy densities, up to 100 Wh/kg, did not overcome certain technological barriers.
Another line of development, which has been worked out during the middle of the 50's, is based on a fuel cell fed, in the most recent embodiments, with air (positive electrode) and a mixture of H2-CO2 (negative electrode), such a mixture being obtained from a methanol reformer integrated with the fuel cell.
The recent introduction of fluorinated ion exchange membranes offering a high conductivity (G. A. Eisman--Symposium on Diaphragms, Separators and Ion Exchange Membrane--Electrochemical Society Proceeding Vol. 86-13 page 156) got closer to the target and, at the same time, emphasized the irrationality of combining a generator system made of a fuel cell plus reformer, (rather rigid as it requires operation at steady load) with an application, the electric vehicle, which in the urban traffic needs peak power outputs during the accelerations but for a good part of the remaining time (deceleration, stops, etc.) it requires a modest or even zero power output.
In order to interface the generator system (battery or fuel cell) with the end-use system (electric vehicle), a hybrid system, battery/battery or fuel cell/battery, has been proposed, where a back-up battery is aimed to supply for short periods the peak power outputs required for the acceleration, while a steady-output generator supplies sufficient energy for the normal operating conditions, while recharging the battery during stops and even during the operation with the unexploited exceeding power.
This concept has already been developed combining a membrane fuel cell to a nickel-cadmium battery or, in alternative to a zinc-air battery, as proposed by the Japanese MITI program for a hybrid acid lead-zinc air batteries system. A different solution may be the development of nickel-metal hydrides batteries as recently proposed with interesting but not so satisfactory results to solve the problems of interfacing the generator to the energy requirements of an electric vehicle (H. Ogawa, M. Ikoma, H. Kawano and I. Matsumoto--to Preprint No. 28 of the 16th International Power Source Symposium-1988).